I went there with my dad for his birthday! I hope the tour is still as good as it used to be (we went pre-COVID). The tour was led by a 30+ year veteran of the factory, and we got to walk right among the work benches and talk to people who were in the middle of making instruments. At the end of the tour everybody got one of the round pieces of wood they cut out to make the sound holes, with the Martin logo burned onto it.

We were able to just walk in, but it looks like you have to pre-register now.